Currently raceSymbolizeCode uses funcline, which is internal runtime
function which crashes on incorrect PCs. Use FileLine instead,
it is public and does not crash on invalid data.

Note: FileLine returns "?" file on failure. That string is not NUL-terminated,
so we need to additionally check what FileLine returns.

Fixes #17190

Change-Id: Ic6fbd4f0e68ddd52e9b2dd25e625b50adcb69a98
Reviewed-on: https://go-review.googlesource.com/29714
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>